Not applicable.
Not applicable.
1. Field of The Invention
This invention relates, generally, to a device and method for determining thermal capacity of overhead power lines.
2. Description of Related Art
Utility companies use overhead transmission and distribution power lines to carry electricity from their generation facilities to industrial and residential customers. As the population grows, so too does the demand for electricity. This increased demand requires a corresponding increase in the capacity of the infrastructure.
One solution to this problem is to build new power lines, but this is a very expensive undertaking due to the extensive labor and materials requirements. It can also be very difficult to obtain all the necessary permits and right-of-way access for the construction of new power lines. This combination of factors has led in recent times to a serious scarcity of power lines. In such a climate, it is extremely desirable to make maximum use of the lines currently in existence.
The capacity of an overhead power transmission line is limited by the heating of the line due to the inherent electrical resistance of the conductor. If a line is operated above its thermal rating, or ampacity, the current dissipated generates a level of heat that is both unsafe and damaging to the conductor. The amount by which the ampacity of a line is exceeded combined with the length of time the overloaded conditions persist determine the seriousness of the ill effects.
One consequence of exceeding the thermal rating of a line is that the conductor will expand beyond its design conditions. Some expansion due to heating is expected, and power lines are designed with a certain level of clearance to accommodate the resulting sag. However, as the temperature rises above the rated value, the increased sag can cause the line to contact objects such as trees, under-built lines, and other foreign objects. Another consequence of exceeding the thermal rating is that the conductor will become so hot that it loses tensile strength due to annealing of the metal. This will reduce the life of the conductor.
Repairing power lines that have been damaged by exceeding their thermal rating is very costly, both in terms of the materials and the lost use of the line. Additional costs are incurred if service to customers is interrupted. These costs include loss of revenue from the sale of power and losses for the customers due to the shutdown of their business. Such an event is also a public relations disaster for the servicing electric company.
Because the limiting factor of a power line is its operating temperature, all factors that affect temperature will also affect the capacity of the line. There are many such factors that have both drastic and subtle effects. For example, the power line dissipates heat by convection at a rate that is highly dependent on both the ambient temperature and the speed and direction of the wind relative to the line. The ambient temperature also affects the rate at which the line dissipates heat by radiation. Rain and other types of moisture can also have a large cooling effect when present. Counteracting these cooling effects, solar radiation adds heat to a line at varying rates depending on the latitude, time of day, cloud cover, and other factors.
Several of the factors mentioned above are difficult to quantify with much precision. For example, the wind speed and direction may be constantly changing, the sun may periodically pass behind a cloud, and rain intensity may fluctuate. Even the most sophisticated weather stations may not provide the quality of data required to accurately predict the rate at which a power line will be cooled by its environment.
Without good data to precisely predict the cooling rate, the worst possible weather effects must be assumed so that the lines are not overloaded. With respect to power lines, the worst weather effects occur on hot, sunny days with no wind. Most of the time conditions are not so poor, however, and the line could actually carry more power while remaining safely below the rated maximum conductor temperature. This uncertainty can result in the lost opportunity to safely transmit up to twenty percent more power over the course of a year than worst case conditions would allow.
Many methods are found in the prior art which try to narrow the gap between the rated capacity and the actual capacity. One method is to calculate the rating using weather conditions for the area as reported by various weather agencies. This is the easiest and least expensive way to increase the line rating, but the uncertainties involved in such an approach require that a large factor of safety be maintained. Weather stations in the vicinity of the line can also be installed and used to provide data. However, most cannot provide accurate readings of wind that drops below two miles per hour. A typical rotating anemometer will stall at these levels, and the bearings in rotating anemometers tend to degrade, increasing their stall speed further and eventually requiring maintenance. Measurement of solar input and precipitation greatly increases the cost of the weather monitoring equipment.
Another method, for which several designs exist in the art, involves measuring the temperature of the line directly. If an accurate reading is obtained, this method successfully enables the user to operate a line near full capacity. However, obtaining an accurate reading is quite difficult due to the extreme electric and magnetic fields that surround power lines. The known devices that are capable of performing these temperature readings are often very expensive, unreliable, or both. In addition, a large disadvantage exists in that when the load is low, the rating accuracy of this method is very poor. This inaccuracy is important because many lines are rated based on contingency loading. In other words, while the normal load may be low, the operator is required to know the rating in the event of a contingency.
Other designs with varying degrees of effectiveness measure either the sag or the strain in the line and extrapolate from that to determine the line temperature. Measuring the sag or strain of the line is also a direct indicator of whether clearance levels are in danger. These methods suffer the same disadvantage as the line temperature sensors in that ratings cannot be calculated at low load. Strain and sag monitors are also expensive. The strain monitor often requires a line outage to install, which is expensive and often impossible at certain times of the year.
U.S. Pat. No. 5,559,430 to Seppa discloses an apparatus that includes a replica of a portion of a power line. Thermocouples are included to measure the temperature of the replica and that of the ambient air. If full sun is assumed, this method allows for the calculation of an effective wind speed and rating. However, if the sun is partially hidden this estimation of the effective wind speed and rating is quite poor. Further, the apparatus is useless at night or other times when there is essentially no sunlight.
It is an object of the present invention to provide a method whereby the thermal capacity of an overhead power line can be accurately determined in real-time.
It is another object of the present invention to provide an apparatus to assist in the practicing of said method.
It is another object of the present invention to make said apparatus more accurate, more reliable, and yet significantly less expensive than those known in the art.
These and other objects are achieved, according to the invention, by an apparatus and method for determining all heat transfer effects present in an area. From that information the thermal capacity of a power line is calculated.
The preferred embodiment of the invention is an apparatus designed to utilize the IEEE 738 equations. The preferred embodiment comprises two cylindrical rods of approximately the same diameter and material properties of the line to be rated. The rods are placed such that the effects of wind, sun, and precipitation on the apparatus mirror those affecting the line. This is most easily accomplished by elevating the apparatus on a pole, separating the rods with a crossarm so they do not shield one another from weather effects, and aligning the rods parallel to the line.
Each of the rods has a thermocouple attached just under its surface near the longitudinal middle of the rod. The rods are of sufficient length so that the cooling out of the ends is insignificant compared to other effects. One of the rods has a resistive heater inserted into it designed to dissipate about the same amount of energy as the line operating at the static rating. A computer then uses the thermocouple readings and attributes of the apparatus to calculate the thermal rating according to the IEEE 738 equations.